The definition of military arrest is very straightforward. As the phrase suggests, this simply refers to being detained or arrested by a member of the armed forces. The United States has long been wary of giving domestic powers of arrest of citizens to the military and has chosen to leave domestic arrests in the hands of non-military law enforcement agencies such as local and state police departments or federal groups such as the FBI or the Drug Enforcement Agency.
Military Arrest in the Armed Forces
Members of the armed forces, whether on American soil or abroad, are subject to military laws, which are enforced by military police and tried by courts martial. An enlisted soldier who violates a military law by being insubordinate to a superior officer, for example, may be subject to arrest by members of the military police. Similarly, crimes committed by soldiers on a military base would be enforced by military police and result in a military arrest.
Military Arrest on Foreign Soil
Another form of military arrest involves the capture and detention of enemies of the United States on foreign soil. Recent examples would be suspected militants or terrorists captured in Iraq or Afghanistan. The United States and the international legal system have special rules on the detention and treatment of enemy soldiers or combatants captured abroad. This has led to controversy for the United States in recent years with respect to detention centers such as Abu Ghraib prison and Guantanamo Bay.
Military Arrest Domestically
Because of a fear of an overly powerful military, the United States has long been wary of giving the military the power to arrest civilians domestically. In 1878, in the aftermath of Civil War Reconstruction, the federal government passed the Posse Comitatus Act, which generally forbids federal military personnel--including members of the National Guard acting under federal authority--from engaging in activities typically undertaken by law enforcement agencies, such as arrests.
The Posse Comitatus Act has several exceptions. For example, Coast Guard units under the authority of a state governor may act in a law enforcement capacity. Additionally, an exception exists for military personal engaged in support roles for the Joint Special Operations Command. Another exception concerns nuclear weapons or materials. The attorney general may request military personnel to supplement civilian law enforcement to respond to certain nuclear threats. Perhaps the most familiar use of the military in a law enforcement capacity is the Insurrection Act, under which the president may order the military to engage in law enforcement activities during certain emergencies.
The Insurrection Act was passed in 1807 with the express purpose of allowing the president to use the military to suppress domestic violence, insurrection and rebellion. Actions undertaken pursuant to the Insurrection Act are exempt from the Posse Comitatus Act. In 2007, the act was amended to add natural disasters and terrorism to the list of situations in which the president may use the military for law enforcement purposes; however, those changes were later repealed and the Insurrection Act has reverted to its 1807 form.